1. Field of the Invention
This invention relates to a reflective liquid display device that ensures a high reflectance and contrast ratio in a range of wide viewing angles.
2. Description of Conventional Art
In general, a reflective liquid crystal display device includes a lower substrate, a reflector, a lower orientation film, a liquid crystal layer, an upper orientation film, an upper transparent electrode, an upper substrate, an optical film and a polarizer. In such general structure, the upper and lower glass substrates are disposed facing each other, and the liquid crystal material is located between them. Phases of the liquid crystals may include nematic and cholesteric. In a nematic phase material, the orientation of liquid crystals can include homogeneous, homeotropic, hybrid and twisted.
A twisted liquid crystal is one in which liquid crystal molecules are aligned substantially parallel to the surface of the substrates but the direction of orientation is twisted in an angle between the substrates. A reflective liquid crystal display device having a twisted nematic liquid crystal mode (xe2x80x9cTNxe2x80x9dmode) generally operates according to the following principles.
When no voltage is applied across the liquid crystal material, light passing through the upper substrate is linearly polarized through a polarizer, and then is circularly polarized through a retardation film. The circularly polarized light then passes through the liquid crystal layer and is converted to linearly polarized light. This linearly polarized light is then reflected from a reflector. The reflected light again passes through the liquid crystal layer and is converted to circularly polarized light, which then passes through the retardation film, and is converted to a linearly polarized light parallel to the polarization axis of polarizer. Such light passes through the polarizer and out of the display device.
In contrast, when a sufficient voltage is applied across the liquid crystal material, circularly polarized light, after passing the polarizer and retardation film, passes through the liquid crystal layer with no change. The light then reflects from the reflector, remaining circularly polarized light. Then, it passes the liquid crystal and retardation film to, becoming a linearly polarized light perpendicular to the axis of polarizer. Such light is absorbed by the polarizer, and does not exit the display device.
Display images greatly depend on optimization of the parameters of each cell component. The important parameters for efficient transmission of light through a reflective liquid crystal display device are the transmission axis angle of polarizer, the optical structure of the retardation film, the thickness (d) and birefringence property (xcex94n) of the liquid crystal layer, and the orientation direction of the orientation film.
FIG. 1 is a schematic diagram illustrating a cell configuration of a conventional reflective liquid crystal display device, where a 90xc2x0 twisted liquid crystal mode is adopted. The device includes a lower substrate 9, a reflector 10, a lower orientation film 11, a liquid crystal layer 13, an upper orientation film 15, a transparent electrode 17, an upper substrate 19, a xcex/4 film 21 and a polarizer 23.
The xcex/4 film 21 is a uniaxial film having a retardation of xcex/4 for optical compensation. In the cell configuration of FIG. 1, the orientation direction 15xe2x80x2 of upper orientation film 15 makes a 90xc2x0 angle with the orientation direction 11xe2x80x2 of the lower orientation film 11. The transmission axis 23xe2x80x2 of polarizer 23 makes a 20xc2x0 angle with the orientation direction 15xe2x80x2 of upper orientation film 15 and a 45xc2x0 angle with the optical axis 21xe2x80x2 of the uniaxial xcex/4 film 21. The xcex/4 film 21 is composed of one uniaxial film.
However, when using one uniaxial xcex/4 film, it cannot consistently produce a xcex/4 retardation over a broad wavelength band of visible light, and thus the display characteristics of the liquid crystal display device are not good.
An object of this invention is to provide a reflective liquid crystal display device with a high reflectance and contrast ratio and a wide viewing angle, while optimizing the parameters of the cell components.
In order to achieve the object, this invention provides, in one embodiment, a reflective liquid crystal display device including a reflector to reflect light impinging thereon; a first orientation film on the reflector, the first orientation film having a first orientation angle; a twisted nematic liquid crystal layer on the first orientation film; a second orientation film on the liquid crystal layer; a first uniaxial film on the second orientation film, the first uniaxial film having a first optical axis angle; a second uniaxial film on the first uniaxial film, the second uniaxial film having a second optical axis angle, and wherein the first and second uniaxial films together operate to change a polarization of incident light between circular and linear polarization; and a polarizer on the second uniaxial film to pass light of a given linear polarization.